1. Field
The subject invention relates generally to implantable cardioverter/defibrillators and, more particularly, to detection of cardiac arrhythmias with subcutaneous implantable cardioverter/defibrillators. Background of the Invention
2. Background
Defibrillation/cardioversion is a technique employed to counter arrhythmic heart conditions including some tachycardias or fast heart rhythms originating in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with electrical impulses or shocks of a magnitude substantially greater than pulses used in cardiac pacing. A variety of shock waveforms are used for both defibrillation and pacing, including truncated exponentially decaying monophasic and biphasic waveform pulses as well as pulses where the waveform maintains a relatively constant current over the duration of delivery to the myocardium.
Defibrillation/cardioversion systems include body implantable electrodes that are connected to a hermetically sealed container housing the electronics, battery supply and capacitors. The entire system is referred to as an implantable cardioverter/defibrillator (ICD). The electrodes used in ICDs can be in the form of patches applied directly to epicardium, or, more commonly, the electrodes are located on the distal regions of small cylindrical insulated catheters that typically enter the subclavian venous system, pass through the superior vena cava, and into one or more endocardial areas of the heart. Such electrode systems are called intravascular or transvenous electrodes. U.S. Pat. Nos. 4,603,705; 4,693,253; 4,944,300; and 5,105,810, the disclosures of which are all incorporated herein by reference, disclose intravascular or transvenous electrodes, employed either alone, in combination with other intravascular or transvenous electrodes, or in combination with an epicardial patch or subcutaneous electrodes. Compliant epicardial defibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and 5,618,287, the disclosures of which are incorporated herein by reference. A sensing epicardial electrode configuration is disclosed in U.S. Pat. No. 5,476,503, the disclosure of which is incorporated herein by reference.
In addition to epicardial and transvenous electrodes, subcutaneous electrode systems have also been developed. For example, U.S. Pat. Nos. 5,342,407 and 5,603,732, the disclosures of which are incorporated herein by reference, teach the use of a pulse monitor/generator surgically implanted into the abdomen and subcutaneous electrodes implanted in the thorax. This system is far more complicated to use than current ICD systems using transvenous lead systems together with an active canister electrode, and therefore, it has no practical use. It has, in fact, never been used because of the surgical difficulty of applying such a device (3 incisions), the impractical abdominal location of the generator and the electrically poor sensing and defibrillation aspects of such a system.
Recent efforts to improve the efficiency of ICDs have led manufacturers to produce ICDs that are small enough to be implanted in the infraclavicular pectoral region, a site allowing access to the subclavian venous system. In addition, advances in circuit design have enabled the housing of the ICD to form a subcutaneous electrode. Some examples of ICDs in which the housing of the ICD serves as an optional additional electrode are described in U.S. Pat. Nos. 5,133,353; 5,261,400; 5,620,477; and 5,658,321 the disclosures of which are incorporated herein by reference.
ICDs are now an established therapy for the management of life-threatening cardiac rhythm disorders, primarily ventricular fibrillation (VF) and also ventricular tachycardia (VT). ICDs are very effective at treating VF and VT, but traditional ICD implantation still requires significant surgery and surgical skill, especially regarding lead insertion into the venous system and lead positioning in the heart.
As ICD therapy becomes more prophylactic in nature and used in progressively less ill individuals, including children, the requirement of ICD therapy to use intravenous catheters and transvenous leads is a major impediment to very long-term management as most individuals will develop complications related to lead system malfunction, fracture or infection sometime in the 5- to 10-year time frame, often earlier. In addition, chronic transvenous lead systems, their removal and reimplantation, can damage major cardiovascular venous systems and the tricuspid valve, as well as result in life-threatening perforations of the great vessels and heart. Consequently, use of transvenous lead systems, despite their many known advantages, are not without their chronic patient management limitations in those with life expectancies of >5 years. The problem of lead complications is even greater in children where body growth can substantially alter transvenous lead function and lead to additional cardiovascular problems and revisions. Moreover, transvenous ICD systems also increase cost and require specialized interventional rooms and equipment as well as special skill for insertion. These systems are typically implanted by cardiac electrophysiologists who have had a great deal of extra training.
In addition to the background related to ICD therapy, the present invention requires a brief understanding of a related therapy, the automatic external defibrillator (AED). AEDs employ the use of cutaneous patch electrodes, rather than implantable lead systems, to effect defibrillation under the direction of a bystander user who treats the patient suffering from VF with a portable device containing the necessary electronics and power supply that allows defibrillation. AEDs can be nearly as effective as an ICD for defibrillation if applied to the victim of ventricular fibrillation promptly, i.e., within 2 to 3 minutes of the onset of the ventricular fibrillation. AEDs, unlike ICDs, only make shock/no-shock decisions, as they are relieved of the burden of needing to deliver complicated tiered therapeutic responses that the ICD encumbers as a consequence of detecting and treating a multitude of rhythm problems with a multitude of therapies. AEDs either shock for a life-threatening event or they do not shock. ICDs, on the other hand, are designed for a variety of interventions and use a different technological approach for arrhythmia detection, redetection, assessment of effectiveness and therapy.
AED therapy has great appeal as a tool for diminishing the risk of death in public venues such as in airplanes. However, an AED must be used by another individual, not the person suffering from the potential fatal rhythm. It is more of a public health tool than a patient-specific tool like an ICD. Because >75% of cardiac arrests occur in the home, and over half occur in the bedroom, patients at risk of cardiac arrest are often alone or asleep and cannot be helped in time with an AED. Moreover, its success depends to a reasonable degree on an acceptable level of skill and calm by the bystander user.
What is needed, therefore, for life-threatening arrhythmias, especially for children and for prophylactic long-term use for adults at risk of cardiac arrest, is a novel combination of the two forms of therapy which would provide prompt and near-certain defibrillation, like an ICD, but without the long-term adverse sequelae of a transvenous lead system while simultaneously harboring most of the simpler diagnostic and therapeutic technological approaches of an AED. What is also needed is a cardioverter/defibrillator that is of simple design and can be comfortably implanted in a patient for many years.
Further, an ICD is needed that can detect various types of cardiac arrhythmias to provide a patient with adequate therapy according to the type of cardiac arrhythmia experienced by the patient. Although ICDs have multiple secondary functions, they primarily have two key functions: detection and therapy of life-threatening cardiac arrhythmias. ICDs constantly monitor a patient's cardiac activity and analyze the cardiac activity to determine the appropriate therapy that should be delivered to the patient. The type of therapies available to be delivered to the patient include pacing, which can be single or dual chamber, therapy to correct slow heart rates or bradycardia called anti-bradycardia pacing, therapy to correct slow and moderately fast ventricular tachycardia (VT) and sometimes atrial tachyarrhythmias, called anti-tachycardia pacing (ATP), and therapy to correct ventricular fibrillation (VF) or high energy shocks. Thus, given the various types of therapies available, it is very important to classify the type of cardiac arrhythmias appropriately.
Detection schemes of cardiac arrhythmias are characterized using two indexes of performance: sensitivity and specificity. Sensitivity generally refers to the ability of the detection scheme or algorithm to accurately detect an abnormal heart rhythm for which the physician desires the device to treat. Specificity generally refers to the ability of the detection scheme or algorithm to not treat rhythms that the physician determines the device should not treat, such as sinus tachycardia. Sensitivity values for VT/VF are typically between 90% to 98%. There is constantly a tradeoff between the ability to detect an abnormal rhythm and the desire to prevent treatment of a normal rhythm. The higher the sensitivity, the more likely the detection algorithm will result in an inappropriate therapy of a normal rhythm and, thus, lower specificity. The higher the specificity, the less likely the device will be able to detect rhythms that should be treated and, thus, the lower the sensitivity. Despite a desire to limit false positive interventions, specificity values are typically only between 70% to 90% in order to not miss therapy of a life-threatening disorder. In practice, there is a constant tension between sensitivity and specificity.
Factors influencing detection algorithm performance include hardware performance, lead electrode placement, electrode design, electrode shape, inter-electrode spacing and the analytical capabilities and design of the detection scheme itself. Because ICDs are battery powered and, therefore, an unlimited supply of power is not available to perform complex and power intensive analytical functions, the energy required to perform the analytical functions is also a factor influencing detection scheme performance.
Another factor that influences the sensitivity and specificity performance of a detection algorithm is the clinical balance between false negatives and false positives for each arrhythmia that the detection algorithm is required to detect. The more arrhythmias required to detect, the more overlap and complexity in the algorithm. For example, one might require a very high sensitivity for fast VT/VF, given its lethality, and therefore accept a lower specificity compared to the relatively benign arrhythmia of atrial fibrillation (AF).
A false negative therapeutic decision results from a detection scheme that calls a bona fide treatable event “normal”. But, depending upon the arrhythmia, some false negatives are more tolerable than others. On the other end of the spectrum is a false positive therapeutic decision that results from a detection algorithm that calls a “normal” event an “abnormal” event, thus inappropriately indicating that therapy should be delivered. False positives for therapy of VT are more clinically acceptable than false positives for rhythms like AF where the former is immediately life-threatening and the later is seldom so.
A false negative can result in a missed arrhythmia and can lead to death, and a false positive may result in an inappropriate shock that will be uncomfortable, but not life threatening. Therefore, it is understandable that detection algorithms are skewed to result in false positives and reduce the occurrence of false negatives to zero for rhythms like fast VT but are more common for rhythms like AF where lower sensitivity is more clinically acceptable. Performance of typical detection schemes in current devices result in approximately 15% to 45% false positives for life-threatening disorders like VF and fast VT.
Typically, ICDs primarily use a rate-based classification scheme. That is, the intervals between successive heart beats are measured and, depending on their values, they are classified as slow, normal, or fast. Slow heart beats are treated with pacing (i.e., anti-bradycardia pacing), when the rate reaches a critically low level, according to a physician's direction via the programming of the device. Programmable parameters for slow heart rates include primarily rate and hysteresis. Normal heart rates are left alone, where normal is usually defined in the range of 40-100 bpm. Fast heart beats are often further classified into three zones with various therapies ascribed to each zone. For example, the lowest zone may have a series of anti-tachycardia pacing (ATP) therapies maneuvers programmed for rhythms like monomorphic VT that may fall in this lowest tachycardia rate zone. The next higher zone may integrate a limited number of ATP attempts with a moderate energy shock therapy of approximately between 5-10 Joules for that zone. Finally, the highest zone may have a scheme programmed to deliver the highest output shock energy for rhythms falling in this zone having VF or VT with the fastest and therefore most life-threatening rates.
Additional rate-based qualifiers have been used to improve the specificity of detection schemes. Examples of such qualifiers include parameters like sudden onset to eliminate false positive detection of sinus tachycardia and heart rate interval stability to eliminate false positive detection of rapid AF for VT. Sudden onset refers to tachyarrhythmias that are a result of a precipitous increase in heart rate as opposed to sinus tachycardia where the higher rates tend to come on gradually. The term stability in detection algorithms usually, but not always, refers to the coupling interval between heart beats as a way to distinguish the characteristically irregular interval-to-interval time sequence of AF from more regular cardiac rhythms like sinus tachycardia or monomorphic ventricular tachycardia where the interval-to-interval time sequence is more regular and rarely varies by more than 30 ms unlike AF where variation by 40 ms or more is the rule. Although typically faster in rate, VF does, like AF, exhibit interval-to-interval instability. It can be distinguished from AF in a detection algorithm mostly by its typically much faster rate but also by overlaying other measures in addition to rate and interval-to-interval stability that result from an examination of the electrocardiographic QRS features.
The term stability, when used in detection algorithms, can have another meaning and refer to electrocardiogram (ECG) QRS signal stability, i.e., the ability of the QRS to have identical or at least very similar signal characteristics on a beat-to-beat basis. In one example of stability, one can examine QRS duration or width, QRS amplitude, QRS slew rate (rate of change of the voltage signal), QRS signal template matching, and/or QRS signal frequency content as an indicator of beat-to-beat stability. VF, for example, under this example of stability would be highly unstable as the QRS is constantly changing in all of the ways indicated above in this paragraph. Confirming the presence of VF, regardless of rate, would aid dramatically in applying a shock/no-shock decision-making process.
Recent developments in QRS morphology measurements involve a scheme in which certain areas within a cardiac QRS complex are measured and compared against a normal set as defined by the physician at the time of implant or as defined automatically by the device during preset follow-up measurement intervals.
There are other issues that enter into the ability of ICDs to detect abnormal heart rhythms depending on the cardiac information that is actually presented to the device's sensing electrodes (which may represent only part of the complete set of cardiac information at any point in time), the ICD energy available to analyze the information, the time required or allowed to make the analysis, and the ability or lack thereof for the device to modify itself over time with respect to changes in the patient's underlying cardiac disease process or even such shorter duration events as changes in body positioning.
With respect to limitations in cardiac information available to the device's sensing electrodes, current devices use a first electrode at the tip deep inside the right ventricle as one half of an electrode sensing pair. The second electrode of the pair can be a shocking coil also in the right ventricle and in close proximity to the first electrode, a second cylindrical electrode approximately 0.5 to 2 cm away from the first tip electrode, or the metallic housing of the device itself. Because the first electrode is in close proximity to cardiac tissue, it is more sensitive to electric fields present in the tissue closest to it and less sensitive to the entire electric field generated by all of the cardiac tissue. This contrast to a subcutaneous ICD sensing pair that has the advantage of a true far-field cardiac signal.
With respect to the energy limitations of current devices, the ability to process and analyze cardiac signals depends on the energy available to the devices. Current devices are limited on the amount of energy available for analyzing signals because the devices must reserve a significant portion of the energy for pacing and shocking over the course of several years. For example, one measure of a device's performance is its ability to filter out unwanted signals, this is called common mode rejection ratio (CMRR). The higher the CMRR of a device, the better able it is to reject signals common to both electrodes. Current devices have a CMRR of about 50 db. Energy is required to achieve higher CMRR values. Another aspect of the analysis requiring energy is the use of a microprocessor with appropriate software inside the device to analyze cardiac information. Continuous running of a microprocessor in an implantable device would result in a rapid battery drain with unacceptably short battery longevity. As a result, current devices reserve a portion of hardware dedicated to crude analysis of the underlying rhythm on a continuous basis and only activate the microprocessor when further analysis is needed. Once activated, the microprocessor, along with its software programming, further analyze the cardiac information, decide on a course of action and, when finished, go back to inactive mode.
For a subcutaneous only ICD, these issues of arrhythmia detection sensitivity and specificity, false positives and false negatives, battery depletion, and algorithm design require significantly different considerations and unique hardware considerations and algorithm design compared to typical ICDs with 1-2 sensing leads in immediate contact with the myocardium.
What is then needed is an implantable cardioverter/defibrillator that has different sensing capabilities to ensure that an ICD using only subcutaneous shocking and sensing electrodes performs at acceptable levels as well as has the ability to adapt itself to changes over time.